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Chapter III

German Intelligence on Allied Convoys in the Atlantic


  1. "Convoy expected."
  2. In relation to cipher compromise.
  3. Convoy chart for blockade runners, December 1943 - January 1944.
  4. North Atlantic convoys.
  5. US-North Africa convoys.
  6. Notable failures to disseminate information on US-Gibraltar Convoys.
  7. UK-Africa convoys.
  8. Analysis of convoy communications made by Atlantic Section; Recommendations submitted to COMINCH.

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  1. "Convoy expected."
  2. The enemy possessed at all times a reasonably clear picture of Atlantic convoys with varying degrees of accuracy as to the routes and day by day plotting. Although independents were not neglected, information on convoys was obviously more important for the U-boat war. Even when group operations were abandoned, knowledge of convoy gathering and dispersal points, ports of entry and departure, and the shipping lanes in coastal areas remained essential. As a rule, convoys were referred to in terms of general course, e.g. "north eastbound convoy," followed by area and date of expected arrival, the area usually being the patrol line itself. In only a few cases, during the fall of 1943, were convoys ever referred to in U-boat traffic by initials (e.g. 1626/16 September 1943, "ON," "ONS" to Group Leuthen). That the enemy knew the correct convoy designators and numbers, however, was clearly show in blockade runner traffic (December 1943 - January 1944), to say nothing of the evidence now available in captured documents.

  3. In relation to cipher compromise.
  4. The "convoy expected" messages came from a background of correlation which included all the sources mentioned in the preceding chapter, with the addition of important punctuation from the reading of convoy dispatches in combined cipher. General convoy intelligence is being discussed here before going into cipher compromised (for which, see the next chapter), because the latter represents a refinement of the "convoy expected" messages, both from the German point of view and from that of the Atlantic Section.

    There is no way of determining at present to what extent information from cipher compromise was interwoven with the standard convoy expectations. For the most part general convoy intelligence and expectations of particular convoys were not sent in the Offizier cipher which was shown by experience to be the normal means for relaying decryption intelligence. There could be no assurance, however, that decryption intelligence had not played a part in patrol line

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    shifts and formations not ordered by Offizier. There is no reason to believe that the Germans were always consistent in observing their own security regulations.

  1. Convoy chart for blockade runners, December 1943 - January 1944.
  2. The most complete single statement of German convoy intelligence ever seen here in German naval traffic came in a series of messages to homebound blockade runners in December 1943 and January 1944. These messages apparently reproduced the enemy's current convoy chart for the North Atlantic, including the Gibraltar lanes. The convoys then at sea were correctly identified both by designators and numbers, and accurate information on convoy cycles, speeds, and general routing was given.

    1. US-UK convoys, general:
      1. Convoys from Halifax to England (abbr. "HX") and Sidney-Canada-England convoys (abbr. "SC") and English convoys (abbr. "ON" or "ONS") generally navigate a great circle from which deviation occurs only if threat from U-boat warrants. Therefore convoys are paired on fixed lanes. The northern portion of these fixed lanes is by far the most navigated." (0105/17 December 1943 ALLE 66)
      2. "HX" and "ON" convoys have a day's run of 204 miles. "SC" and "ONS" a day's run of 180. "HX" and "ON" convoys run at intervals of 6 and 7 days alternately, "SC" and "ONS" at intervals of 13 days.
      3. Stragglers are to be expected after every convoy. They are routed on constantly changing courses." (0253/17 December 1943 ALLE 67)

    2. US-Gibraltar convoys, general:
    3. Designations: "UG" = US to Gibraltar
      "GU" = Gibraltar to US
      Add "S" for slow and "F" for fast
      (1054/22 January 1944)

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      Cycle: 10 Days
      Speed: 204 miles per day (8.5 knots) for "S"
      (0854/5 January 1944 ALLE 91)
      Route: Between 32°N and 36°N from 63°W to Gibraltar
      (0854/5 January 1944 ALLE 91)
      Escort: ... varies. Details not exactly known here. Far ranging reconnaissance by aircraft and close escort by destroyers and destroyer escorts must be assumed.

    1. Dead reckoning for particular convoys.
    2. On 16 December 1943 blockade runners Osorno and Alsterufer received dead reckoning estimates for the following convoys: HX 270, SC 149, ON 215, ONS 25, and UGS 27. On the 18th, England Mediterranean convoys KMS 36, MKS 33, and MKS 34 were added. (1900, 1933, 1948, 2013/16 December 1943 DAN 7-10; 1013, 1037, 1102/18 December 1943 DAN 3335).

      Dead reckoning plots on GUS 26 and UGS 30 were sent 5 January, (0854/5 January 1944 ALLE 91). The convoy identifications were correct. Dead reckoning estimates for US-UK convoys were given in terms of successive "standing lines." For example, convoy HX 270 was plotted for 18 December as being somewhere along a line extending from 51°N - 36°W to 43°N - 35°W. The "standing lines" for HX 270 and SC 149 ran approximately from the standard eastbound convoy route "B" on the north to standard route "C" on the south and did in fact lie across the route taken by these convoys. Had any one of the "standing lines" west of 30°W been occupied by U-boats after the fashion of the preceding winter, contact would have been made at about the estimated time. Group Rugen, then in the area east of 30°W, was informed of these "eastbound" convoys,

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    but, strangely enough, the timing given to the U-boats was not as accurate as that given to the blockade runners. The plotting of ON 215 and ONS 25 was rather poor. GUS 26 and UGS 30 were plotted with fair accuracy in the area between the Azores and Bermuda.

  1. North Atlantic convoys.
  2. Accumulated evidence indicates that convoys which were not "expected" by U-boats were simply those in which the enemy could not take an operational interest. As the North Atlantic began to fill up with U-boats in January 1943, the number of "convoy expected" messages increased, accompanying the formations of lines for practically every major eastbound convoy from the middle of January to the end of May. On the whole a high standard of accuracy was maintained.

    Convoy diversions were sometimes learned from decryption in time to rearrange U-boat patrol lines appropriately. In addition, the large number of U-boats and the pattern of their arrangement in groups tended to negate convoy diversions by covering the major possible diversion routes. Contacts were thus made by U-boats other than those for which the convoy was originally intended. German knowledge of the entire convoy situation was in this way constantly clarified and amplified.

    During the summer of 1943 there was no check on the convoy plot such as had been furnished by U-boat operations. Despite this lack, convoy expectations began again promptly and accurately with the resumption of the North Atlantic offensive in September 1943. More "convoy expected" messages marked the unsuccessful campaign of 1943-1944 than the campaign of the preceding winter. From an average of about 7 a month during the winter campaign of 1942-1943, these messages rose to an average of about 10 per month during the following winter.

    This increase signified the lack of success, reflecting not only a carry-over of unused patrol lines but also the willingness of patrol lines to take westbound as well as eastbound convoys. Furthermore, convoy intelligence assumed a new function in 1943-1944 which had been unheard of in the North Atlantic during the preceding winter; warning to U-boats of the possibility of encountering carrier borne aircraft and

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    other forms of anti-submarine activity. Of the 38 HX an SC convoys which sailed for England from 13 September 1943 to 22 February 1944 no less than 34 were referred to in U-boat; of the 35 ON and ONS convoys during the same period, 27 were mentioned.

    There are several instances in which U-boat Command showed a knowledge that could not have been gained simply from convoy cycle plotting, quite apart from the presumed and confirmed cases of cipher compromise. For example, when Group Coronel was formed in December 1943 to operate against "a slow westbound convoy." Command must have known not only that ONS 24 and ON 214 were not proceeding on similar routes, as had been customary, but also that it was ONS 24 which was taking the northerly route. The insight which the Germans gained solely by analysis of the heavy volume of stereotyped combined cipher traffic on US-UK convoys probably found supplementary information in local British "low-grade" communications.

  1. US-North Africa convoys.
  2. Although the German navy had no advance information the North African landings in November 1942, they had no difficulty in building up a knowledge of US-North African convoys from agents in the Gibraltar area, traffic analysis, and GAF reconnaissance. In addition a large amount of convoy radio traffic in the Mediterranean was sent in combined cipher. U-boat group operations functioned with a smoothness comparable to that on the US-UK lanes, but no lack of intelligence seems thereby indicated. There are, however, the possible failures of intelligence which are discussed in paragraph 6 below.

    1. To summer 1943. Operational intelligence.
    2. During the only period in which U-boat groups kept vigil on the US-Gibraltar lane (winter 1942-1943) at least 4 eastbound convoys were "expected," one of the UGS 6, on the basis of cipher compromise. Only one westbound convoy, GUS 4, is known to have been awaited; cipher compromise also played a part in this case. The best operations, with the possible exception of UGS 6, seem to have resulted from accidental contacts. Group Delphin's destruction of the tanker

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      convoy TM 1 in January was due to an early sighting by a Uboat bound for the Trinidad area. COMSUBs recognized the target, however, as the first Trinidad-North Africa convoy. When U-boats (Group Trutz) returned to the US-Gibraltar lane in June 1943, after an absence of two months, they were certainly provided with good intelligence; in fact the formation of Group Trutz must have been due in large part to compromise of Flight 10's routing dispatch. The enemy was also aware of GUS 7A and, later, of GUS 8A, but did not show any realization in traffic of UGS 9 which was proceeding near Flight 10.

    1. Interest increases in winter 1943-1944.
    2. Information on US-Gibraltar convoys did not reappear in U-boat traffic until the fall of 1943, when U-177 (Gysae), returning from an extended patrol off South Africa, was given dead reckoning positions for UGS 18. The positions were a good 10 degrees ahead of the convoy's progress, indicating a German plotting at 10 to 10.5 knots whereas the convoy was making 9. Gysae received the information primarily for warning. Warning seemed the purpose of subsequent dissemination of intelligence on these convoys, which increased noticeably during the winter. In December dead reckoning plots on convoys UGS 26 and GUS 23 were sent out to five U-boats which were crossing the lane.

      The positions were much nearer the truth than those for UGS 18 had been, but they did not show any exact knowledge of the standard route. While not precise, they were still sufficiently accurate to have effected contact by a suitably disposed patrol line. From January to March 1944 each successive UGS and GUS convoy (UGS 27-34 and GUS 26-32) was referred to in terms of Gibraltar arrival or departure for benefit of U-boats trying to enter the Mediterranean. Thereafter such information was disseminated only on the few occasions when U-boats were ordered to individual patrols in the Gibraltar approaches.

  1. Notable failures to disseminate information on US-Gibraltar Convoys.
  2. One assumes that the Germans kept a current plot on UGS-GUS convoys in which some confidence was felt. It is therefore difficult to account for certain failures to warn supply

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    submarines or to change their rendezvous positions during June, July and November 1943. Two supply submarines were sunk and one endangered in positions which either show poor convoy intelligence or a failure to correlate and use such intelligence as was at hand. Although no dead reckoning positions farther west than 37°W were ever given to the Uboats, blockade runners had been furnished with dead reckoning positions on UGS 30 and GUS 26 as far west as Bermuda. The latter had been originated by a non-U-boat section of the Navy (1 SKL), but it is hard to believe that U-boat Command (2 SKL/BdU op) did not have access to all available information and that it was not capable of plotting convoys all the way across with operational accuracy, or what was deemed operational accuracy.

    1. Case of refueler Czygan's (U-118) sinking 1410Z/12 June 1943, in 30°49'N - 33°49'W by TG 21.12 (Bogue).
    2. Plans for Czygan's refueling station in 30°45'N - 33°40'W were announced in U-boat traffic on 31 May. On 12 June he was sighted on the surface and sunk within six miles of this position. No change had been ordered in his rendezvous assignment despite German knowledge that between 31 May and 12 June considerable aircraft protected Allied shipping had passed through this area. At no time during this interim did Command show any real awareness of the true situation. He did know about Flight 10 and GUS 7A but he apparently knew nothing of UGS 9 or TG 21.12.

      Czygan was warned late on 5 June that an eastbound convoy with aircraft escort could be expected on the 7 June, but this was after the Bogue had attacked the Trutz line. Command had Flight 10 in mind and not UGS 9. The danger from UGS 9 was much greater than that from Flight 10, for the latter had altered course to the north after passing through the Trutz line on 5 June while TG 21.12 had turned south to cover UGS 9, which was proceeding along 29°30'N.

      Schnoor (U-460), another supply submarine, was in the immediate vicinity. Command showed no awareness of UGS 9 until after Manseck (U-758) had accidentally sighted a convoy and had been so heavily bombed for his trouble that he prepared Command for the loss of his boat, after having explained that he had been attacked by carrier borne planes.

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      Manseck pulled off to the south. Not until then were the two supply submarines warned that Manseck's sightings were probably on an eastbound convoy and that they should watch out for carrier aircraft while going to Manseck's rescue. By following Manseck's southerly course Schnoor and Czygan were drawn out of harm's way. But after Manseck had been found, still afloat, and turned over to Schnoor's custody, Czygan returned to his original rendezvous assignment - in time to meet TG 21.12 on its return sweep.

      It should be pointed out that the area south of the Azores had been used for refueling rendezvous' many times in the past without mishap. One might also argue that Command had every reason to believe the three to four day old wake of a convoy a safe place for a refueling rendezvous. Czygan's sinking, however, surely indicates, besides ignorance of UGS 9, that Command was unprepared for the offensive nature of the task group that might engage in free maneuvers of its own.

    1. Case of refueler Metz (U-487) sinking 13 July 1943 in 27°15'N - 34°18'W by TG 21.12.
    2. The important nature of Metz's assignment in July has been dealt with elsewhere. His loss was an irreparable misfortune. One might then expect that the best of German intelligence would have concentrated on his safety. Yet he had not quite arrived at his assigned rendezvous position, 27°09'N - 33°27'W, when he was sunk 131700Z July. Convoy and Routing's plot for GUS 9 at 132000Z was 27°01'N - 33°39'W, within nine miles of the rendezvous position assigned to Metz. In blocking the US-Gibraltar lane to the north of 30°N with Group Trutz, U-boat Command had contributed materially to the southerly routing of GUS 9 which had brought TG 21.12 to Metz.

    3. Case of refueler Bartke (U-488) November 1943.
    4. The loss of U-118 and U-487 may have started the policy of warning U-boats when crossing the US-Gibraltar lane, for this type of message was not seen during the summer of 1943. Gysae was warned in September. Nevertheless, Bartke was twice seriously endangered by convoys during his November cruise without receiving any warning from Control. While passing from one rendezvous position to another in the area

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      mid-way between Bermuda and the Azores he was surely within 60 miles of GUS 19 on the evening of 7 November. Again on 9 November he crossed UGS 23's path not many miles ahead of that convoy. Bartke later reported that he had been depth charged and damaged on 8 November.

      Approaching Biscay on his return cruise, Bartke was given a dead reckoning position on a Gibraltar-England convoy. Warning messages were not sent because of a near collision on the German U-boat chart; U-boats were warned, when warned, even on a remote chance of encounter. For one thing, Command's day by day knowledge of a U-boats position was not necessarily accurate. Hence it seems reasonable to suppose that Bartke would have been informed had the German chart shown any convoys at all between Bermuda and the Azores.

    1. Comment on German intelligence US-Gibraltar convoys, August and September 1943.
    2. As of 15 September 1943, U-boat Command gave the following account of US-Gibraltar convoy routes in Operational Order #56 (Nemo document):

      "The convoy routes lie between 30°N and 40°N latitude; the convoys coming from America travel the southern part, those going to America in the northern part of the lane. Extensive detours, particularly to the south, after attacks on this traffic."

      On 16 August 1943 Current Order #11 carried this statement to all U-boats at sea:

      "For the protection of America-Gibraltar convoys, one or more carriers are located in the area CD (from 34° to 43°N - 26° to 35°W) and DF (from 26° to 34°N - 35° to 43°W). Many repulsed attacks and the unexplained loss of several submarines homeward bound testify to this. In sum, carrier-based aircraft can always be counted on in the whole sea area between Gibraltar, New York and 25 degrees North. All the machines, even carrier-based planes, are probably fitted with radar ..."

      One assumes that the September information on convoy routes was available in June and July. Current Order #11, however, seems to

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      speak directly from the costly experiences of June and July. And yet Operational Order #56 declares that:

      "Since the spring of 1943 the enemy has assigned auxiliary carries to the area between 25°W and 40°W, which is otherwise hard to patrol. These are escorted by from 2 to 4 destroyers, watch over the area mentioned and in case of U-boat attacks come to the aid of the endangered convoys."

      Apparently the detailed implications of this paragraph were not anticipated, or, if anticipated, were left blank to be filled in by experience.

  1. UK-Africa convoys.
  2. At times inbound and outbound U-boats were informed of convoys on the UK-Gibraltar lane, either for purposes of operation or for warning. During the period from April to June 1943, five Offizier messages were sent to U-boats along the northwest African coast informing them of dead reckoning positions on UK-African convoys. These cases should be studied here to a limited extent only since the convoys were entirely British and no convoy files were available. It was later learned that GC&CS presumed these to be instances of compromise, probably derived from dispatches sent in Combined Cipher #3. (see Appendix 13 on "Cases of Presumed and Confirmed Compromise of Allied Communications 1943-1945)

  3. Analysis of convoy communications made by Atlantic Section; Recommendations submitted to COMINCH
  4. Because of the renewed persistence of convoy intelligence in German traffic in the fall of 1943 the Atlantic Section felt obliged to follow up its studies on cipher compromise by an examination of convoy communications procedure and habits. It was of course recognized that the tremendous convoy undertaking could not in practice satisfy all demands of security theory, but the apparent ease with which the German communications intelligence organization maintained its hold on Allied convoys, particularly US-UK, suggested that some improvement could be made in our own communications, quite apart from the introduction of Naval Cipher #5, which had immediately followed the demonstration of extensive compromise in Cipher #3.

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    1. Findings.
    2. It was found that standard procedure made the task of enemy traffic analysts easy. In addition to the heavy reliance on combined cipher, tables "M" and "S", with the subsequent overburdening of a weak system, the timing of many radio dispatches concerned with each convoy observed a pattern as regular as that of the sailings themselves. In the case of HX-SC convoys during the fall of 1943 the ocean route was broadcast by CINCCNA approximately four days prior to each sailing. On the day of sailing and at a set interval after the sailing, each convoy was again announced in the fixed pattern messages from ports of departure and from the authorities concerned with escort relief at the ocean meeting point. The ocean meeting point usually gave rise to exchanges of dispatches between escort commanders and shore commands. A convoy diversion would be followed immediately by a BAMS broadcast. In the case of US-Mediterranean convoys correct identifications in blockade runner traffic had aroused particular interest here, but intensive study of available data had failed to prove cipher compromise.

      The investigation of UGS-GUS communications for the period from November 1943 through January 1944, however, did show beyond question that traffic analysis of the Mediterranean end alone should suffice for maintaining and correcting knowledge of the convoy cycles. A surprisingly large amount of fixed pattern radio traffic was broadcast in the world-wide table of the combined cipher for the information of practically every Allied command of any description in the Mediterranean area. These dispatches followed a fixed timing in relation to the convoy's progress. The check made by agents at the Straits or by GAF reconnaissance seemed almost unnecessary as far as major convoys were concerned.

    3. Recommendation.
    4. The results of these studies were made available to U.S. Navy Communications Security and COMINCH. To eliminate the dependence on combined cipher in the Mediterranean the introduction of more cipher machines was hastened. For US-UK convoy communications certain remedial measures were drawn up and submitted to the Director of Naval Communications, who incorporated them in a memorandum for COMINCH (F-34). It was recommended that:

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      "a. All shore authorities and ships in Western Atlantic be notified by dispatch that messages should be sent in "A" or "M" rather than in "M" or "S" Table when the 10th Fleet C&R is the only US authority included in the address.

      b. All shore authorities and escort vessels in the Western Atlantic be notified by dispatch that messages addressed to US authorities in addition to 10th Fleet C&R, but of no immediate action is required, be sent as in (a), above, with appropriate passing instructions in the text.

      c. CINCCNA make greater use of aircraft to carry home messages for ships and that such aircraft report last location of convoy rather than escorts break radio silence to report positions.

      d. Admiralty modify convoy instructions particularly to stress the great danger of breaking radio silence and using homing procedure in the mid-Atlantic.

      e. Admiralty and CINCCNA examine the standard reports required by shore authorities and escorts in connection with convoys with a view to eliminating unnecessary reports or stereotyped patterns."

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